This invention relates to microwave frequency power amplifiers used in microwave frequency transmitters and in particular to an apparatus and method of achieving improved efficiency and linearity with a microwave Doherty amplifier.
New commercial and military telecommunications systems over the past few years have shown a trend toward using digital modulation techniques. These digital systems require the capacity to handle a high density of carrier frequencies in order to remain cost effective. The additional trend toward space based systems imposes high efficiency and weight minimization constraints. Power amplifiers represent an important design challenge as they must conform to the above specifications. Unfortunately, high efficiency in power amplifiers has been difficult to attain for a large number of carrier frequencies. In fact, this is a direct trade-off in conventional amplifier design.
In conventional power amplifiers, it is important to note that the amount that the input drive level must be reduced from the saturation level is directly proportional to the number of carrier frequencies being amplified; also, there exists a direct trade-off between linearity and efficiency. An increased number of carrier frequencies compounds the efficiency problem since the amplifier enters gain compression earlier (at a lower drive level) than in a single tone case due to the increased peak-to-average ratio within the exciting signal. Previous techniques developed to overcome this problem require complicated circuit designs that are large and difficult to implement. None of these techniques were suitable for implementation in a space based communication or radar system due to their size, weight, reliability or complexity.
The Doherty amplifier was first suggested by W. H. Doherty in 1936 and is discussed in a technical paper entitled "A New High Efficiency Power Amplifier For Modulated Waves," W. H. Doherty, Proceedings of the Institute of Radio Engineers, Vol. 24, No. 9, September 1936. Originally intended for use in low to medium frequency amplitude modulated broadcasting transmitters, the suggested scheme can be modified and updated to solve the aforementioned amplifier difficulties. In a conventional amplifier there is a direct relationship between efficiency and the input drive level. Therefore, high efficiency is not attained until the RF input power becomes sufficiently high to drive the amplifier into saturation. Since in multi-carrier communication systems an amplifier must remain as linear as possible in order to avoid intermodulation distortion, this region of high efficiency cannot be used. A theoretical Doherty amplifier does not have the same relationship between efficiency and input drive level; hence a high efficiency linear region is created.
The Doherty amplifier scheme achieves high linear efficiency by having one Class B amplifier operated at the point where the output begins to saturate, and where the highest linear efficiency is obtained. A second amplifier is used to affect the first so that overall linearity can be maintained as it is driven beyond this point. A Doherty amplifier circuit using two vacuum tubes and two phase-shift networks is shown in FIG. 1 which is from the "Radio Engineering Handbook," Keith Henney, Editor-in-Chief, Fifth Edition, McGraw-Hill Book Company, 1959, pp. 18-39. Also, the Doherty amplifier is described in U.S. Pat. No. 2,210,028, issued in 1940 to W. H. Doherty. A first electron discharge tube conducts continuously and is termed the carrier tube whereas a second electron discharge tube does not conduct for the whole period of each cycle of a modulation signal and is termed the peaking tube.
Class B amplification of radio-frequency waves, having been known and used in the design of high power RF amplifiers through the early 1930s, was first improved by the technique published in the Proceedings of the Institute of Radio Engineers, Vol 23, No. 11, by H. Chireix in 1935, titled "High Power Outphasing Modulation." The Chireix "outphasing" system splits the output of a radio frequency oscillator into two equal amplitude and equal phase signals. These signals then pass into phase-shifting networks of 90 degrees leading and 90 degrees lagging. An amplifier in each leg modulates the phase-shifted signal when driven by a low-level phase modulated signal derived from the intended modulation and the original carrier frequency wave. It is necessary that the directions of the phase modulation imposed on the delayed RF carriers be opposite to each other. Therefore, with the above condition in mind, each signal represents an RF wave 90 degrees out-of-phase with respect to the original carrier with an oppositely directed phase "wobble" (the intended modulation) on each of them. Both signals are used to drive a pair of conventional high-efficiency power amplifiers operating in Class C. The amplifiers are adjusted in exactly the same manner so as to preserve the insertion phase of the stages relative to each other. Output from each amplifier is delayed by an additional 90 degree network the outputs of which are connected directly together with the load, being taken from a mid-point so formed.
Vector summation of the phasors at the common point shows that, provided all of the conditions above are met, a linear amplifier of high efficiency exists at maximum output power (also, maximum modulation). In the Chireix outphasing system, the efficiency degrades quickly as the output power or the modulation index is reduced since the phase relationships stated do not allow the reactive components generated at the final combining network to completely cancel presenting a resistive load except in the case where maximum power at maximum modulation is being produced. The Chireix system also has the disadvantage of requiring numerous critical tuning adjustments while the bandwidth and linearity of the phase modulation circuit is of great concern in a wideband application. A Doherty amplifier circuit achieves better overall efficiency as a function of drive, eliminates these critical adjustments to a large extent, simplifies the realization of the overall transmitter, and does not have an implicit bandwidth limitation like the Chireix design.
A more modern method, that upon first glance looks like a Doherty amplifier, is the balanced amplifier technique of Kurokawa as described in the article "A Wide-Band Low Noise L-Band Balanced Transistor Amplifier," R. S. Engelbrecht and K. Kurokawa, Proceeding IEEE, Vol. 53, pp. 237-247, March 1965. An input excitation is fed to two identical amplifiers ninety (90) degrees out-of-phase from a quadrature hybrid network. The output signals from the two amplifiers are recombined in another quadrature hybrid network to form an in-phase single-ended output signal. Two major differences from the Doherty amplifier concept are as follows: (1) No provision is made to alter the bias points of the two amplifiers at the input as is done in the Doherty. Indeed, in the balanced amplifier this is undesirable as the mis-match seen by the hybrid network would result in increasing losses in the termination resistor of the network; and (2) The output of each amplifier combined by the action of the hybrid network precludes the two amplifiers interacting so as to "pull" or influence the load seen by one amplifier over the other as in the Doherty. In fact, the direct interaction of the two amplifiers within a three-port Doherty network accounts for all of the observed efficiency enhancement of the Doherty amplifier and is central to its operation.
The manner in which the invention deals with the disadvantages of the prior art to provide improved efficiency and linearity with a microwave Doherty amplifier will be evident as the description proceeds.